It is desired to connect various types of stranded copper wire to the steel electrode pins of an initiator assembly containing combustible material in order to provide a means to conduct electric current to the initiator assembly in order to trigger the assembly. In addition to providing a good electrical connection, the connection must not fail when subjected to a pull test load of about 35 pounds. It is therefore desirable to involve each strand of the wire in the connection in order to more effectively distribute the load. It is also desirable to limit the amount of total heat which is applied to the electrode pins during welding in order to reduce the risk of premature initiation of the combustible material.
The previous method of connecting stranded wire to an electrode pin of the initiator assembly was by arc percussive welding. In this method, the materials to be joined are heated by an arc produced by rapid discharge of electrical energy stored in a capacitor. Force is percussively applied during or immediately after the electrical discharge, forcing the wire and the pin together. A result of percussive arc welding is the formation of an appreciable ball of material in the area of the resulting joint due to displacement of the melted material resulting from the application of force. In addition, the resulting weld area tends to be porous, as a result of the trapping of gas in the welded area.
A drawback of arc percussive welding is that the arc starting point may be inconsistent, with the result that the welds are of inconsistent quality. In addition, inconsistent quality results from limitations in precisely locating the work pieces, as well as controlling the timing and mechanical motion used in imparting the percussive force. These factors tend to make automation of the process difficult.
Lasers have been used as a heat source in several applications. Multiple semiconductor lasers have been used to solder surface mount integrated circuits to printed circuit boards, as in U.S. Pat. No. 4,963,714 to Adamski et al. Laser energy is used to solder a wire to a conductor in U.S. Pat. No. 3,665,367 to Keller et al., and is also used to fuse a crimped conductor to adjacent wire strands in U.S. Pat. No. 4,966,565 to Dohi. Two laser beams are used in the brazing operation disclosed in U.S. Pat. No. 4,714,815 to Swarts et al. Swarts discloses the use of two laser beams focused on a mounting surface on either side of a fine wire. The lasers are used to heat the mounting surface, without directly heating the wire, so that the surface material is melted and wicked up over the wire to form a joint.
Electrolytic tough pitch copper has been successfully welded to high conductivity aluminum using a process similar to the arc percussive welding described above. Instead of an electric arc, a single pulse from a laser beam is directed at the point where the materials are forced into contact by application of pressure in U.S. Pat. No. 4,224,499 to Jones. The brittle intermetallic compounds that are produced as a result of melting the aluminum and copper are forced into a flash region exterior to the joint by the contact pressure. In U.S. Pat. No. 4,774,394 to Lemke a laser aimed down the axis of a wire is used to weld the end of the wire to a metallic fixture.